1. Field of the Invention
The present invention relates to a recording medium, and more particularly, to a thermally stable perpendicular magnetic recording medium.
2. Description of the Related Art
Generally, a parallel magnetic recording medium on which data is recorded in parallel along tracks is used in a hard disk drive (HDD).
In the case of a parallel magnetic recording medium, there is a limit due to the relation between the thickness of a parallel magnetic recording layer and the length of a parallel magnetization area, which is used to record unit bits. In other words, the length of the parallel magnetization area should be larger than the thickness of the parallel magnetic recording layer.
In order to record data on a parallel magnetic recording medium at a high density, the length of a parallel magnetization area should be reduced. However, in the case where the length of the parallel magnetization area is reduced, the thickness of the parallel magnetic recording layer should be reduced more. As a result, there is a limit in the improvement of the recording density of a parallel magnetic recording medium.
In a perpendicular magnetic recording medium, data is recorded in perpendicular magnetization area relative to tracks.
The structure of a conventional perpendicular magnetic recording medium is shown in FIG. 1.
Referring to FIG. 1, a conventional perpendicular magnetic recording medium includes a substrate 10, a perpendicular alignment underlayer 12 for perpendicularly aligning a magnetic easy axis, a perpendicular magnetic recording layer 14, a protection layer 16, and a lubricant layer 18.
In order to thermally stabilize such a perpendicular magnetic recording medium, the thermal stability of the perpendicular magnetic recording layer 14 should be secured. The thermal stability of the perpendicular magnetic recording layer 14 may be verified using a thermal magnetization reversal rate R, which is represented by Equation 1.
                    R        =                              f            0                    ⁢                      exp            ⁡                          (                              -                                                      E                    B                                                                              k                      B                                        ⁢                    T                                                              )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, f0 denotes an attempt frequency of generating a magnetization reversal, EB denotes an energy barrier, which is the energy for generating the magnetization reversal, kB denotes a Boltzmann constant, and T denotes a Kelvin temperature.
In general, in order to preserve the data recorded in a magnetic method for more than 10 years,
      E    B              k      B        ⁢    T  of Equation 1 should be larger than 60, in other words,
            E      B                      k        B            ⁢      T        >  60.Here, EB is known as KV, in other words, EB=KV, wherein K denotes a magnetic anisotropy constant of a magnetic layer on which data is recorded and V denotes a grain volume of the magnetic layer.
When the thickness of a perpendicular magnetic recording layer in a perpendicular magnetic recording medium having a large K is larger than a predetermined thickness, the energy barrier does not follow the KV anymore, but varies non-linearly. That is disclosed by Dieter Suess, Thomas Schrefl, and Josef Fidler, in “Reversal Modes, Thermal Stability and Exchange Length in Perpendicular Recording Media”, IEEE Transactions on Magnetics, VOL. 37, NO. 4, pp. 1664-1666, 2001.
Accordingly, a perpendicular magnetic recording medium, which is designed based on an energy barrier of KV, may be thermally unstable because the energy barrier may be smaller than KV. As a result, the time in which data is preserved may be reduced.